The primary assimilation of carbon dioxide into organic matter is catalyzed by the most ubiquitous protein found on earth, D-ribulose 1,5-bisphosphate (RuDP) carboxylase. The enzyme is unique in that it can also catalyze an oxygen fixation reaction. Moreover, in eukaryotic cells, the enzyme is composed of nonidentical subunits, each of which is synthesized in separate compartments. This proposal is directed towards elucidating the molecular basis for catalysis and will also be concerned with how this key enzyme is assembled in eukaryota. Previous studies have shown that a lysyl residue may be important in catalysis. Thus part of the proposed work will seek to extend these preliminary studies, using several probes, with the goal of definitively identifying the chemical moieties at the catalytic site of this enzyme. Moreover, experiments will be performed to attempt to selectively modify either the carboxylation or oxygenation function of this enzyme. In all such studies, structure-function relationships will be emphasized to further aid in defining the chemical basis of catalysis. Techniques of protein and peptide isolation, protein modification, and enzyme kinetics will be employed in this phase of the research. In eukaryotic organisms, RuDP carboxylase is found in specialized organelles, where it comprises up to 60 percent of the total soluble protein. The large catalytic subunit is encoded by organelle (chloroplast) DNA and the small (regulatory) subunit is encoded by nuclear DNA. The transport of the small subunit and subsequent assembly of the two subunits in the organelle, the relationship of this assembly and the mechanism of synthesis of each subunit will be studied. Organelle isolation procedures and protein synthesis techniques will be used in these experiments.